Method for determining force characteristic of a device for magnetically holding workpieces

ABSTRACT

A method for determining force characteristic of a device for magnetically holding workpieces placed on a top plate (3) of the device, wherein a workpiece (1) cooperating with a source of magnetic field of the device is connected to a force pick-up (2) and is caused to move along the top plate (3), along a test line (4). The source of magnetic field remains permanently energized during the tests, the workpiece (1) is placed in parallel with the top plate (3) with a clearance which depends on sensitivity of the force pick-up (2), and is caused to move along the test line (4) continually while maintaining the amount of the clearance substantially unchanged.

FIELD OF THE ART

The invention relates to machine tool engineering, and in particular, itdeals with methods for determining force characteristic of a device formagnetically holding workpieces.

STATE OF THE ART

The main characteristic of devices for magnetically holding workpiecesis a force characteristic, i.e., distribution of the force of attractionof workpieces over the surface of a plate which makes it possible toevaluate capabilities of the device and quality of its manufacture.

It is important to reveal, in such distribution of force, values andzones of minimum attraction forces to be used in setting forth machiningconditions for workpieces being held, and also the character ofdistribution which determines magnetic deformations of parts, and hence,influences accuracy of their machining.

The force tests are the main method for evaluating quality of a jig thatmakes it possible to detect flaws of component parts and assembly.

Widely known in the art is a method for determining force characteristicof devices for magnetically holding workpieces, comprising:

placing a test sample or a workpiece at a first control point on theplate surface with the device turned-off;

turning the device on;

stripping the test sample from the device in the turned-on positionthrough a dynamometer in the direction at right angles to the surface ofthe device and recording the stripping force;

demagnetizing the device;

placing the test sample at a second control point, and so on.

Stripping off the test sample is effected in a succession along a testline at a pre-set spacing. Thus, a maximum advance where a user can gettrustworthy and sufficiently detailed information on the distribution ofthe forces of attraction along the surface of the device is about 1/7 ofthe test diameter.

The test results are plotted in the form of a broken line, numbers ofcontrol points being plotted on the abscissa and values of attractionforces at these points being plotted on the ordinates, and the resultantpoints are interconnected by a broken line.

It will be apparent that the shorter the distance between the controlpoints being interconnected, the closer is the resultant broken line tothe actual force characteristic of the device. At the same time, anincrease in the number of control points results in a substantialincrease in the test time.

All preparatory and measurement operations connected with a forcecontrol at one point of the plate take about three minutes. Bearing inmind that such control should be effected at more than 170 points for anelectromagnetic plate of a size 320×1250 mm, it will be understood thatthe control time of such a device is commensurable with the time neededfor its manufacture (the force test is carried out for over 8 hours). Itis obvious that such tests will take a much longer time for plates oflarger size as 630×2500 mm.

At the same time, even such long tests will not always allow actualvalues of minimum attraction forces to be revealed because of theirdiscontinuous nature as control points may not coincide with zones ofapplication of minimum forces of attraction.

Therefore, the prior art methods for controlling force characteristic ofholding devices take a long time and do not make it possible to obtain acomprehensive force characteristic of the device. It should be notedthat reducing the test time (reduction of the number of control points)results in a lower reliability of the test results and, vice versa, animprovement of reliability of the control results (an increase in thenumber of control points) calls for a substantial increase in the testtime.

SUMMARY OF THE INVENTION

The invention is based on the problem of providing a method fordetermining force characteristic of devices for magnetically holdingworkpieces which allows a full force characteristic of the device to beobtained with a short test time.

This problem is solved by a method for determining force characteristicof a device for magnetically holding workpieces installed on a top plateof the device, comprising connecting a workpiece cooperating with asource of magnetic field of the device to a force indicator and causingthe workpieces to move along a test line, according to the invention,the source of magnetic field is held permanently energized during thetests, the workpiece is placed in parallel with the top plate with aclearance therebetween which depends on sensitivity of the force pick-upand is caused to move continually along the test line while maintainingthe amount of the clearance substantially constant.

The method for determining the force characteristic of devices formagnetically holding workpieces by continuously providing a magneticforce of attraction between the workpiece and the top plate according tothe invention makes it possible to obtain a comprehensive forcecharacteristic of the device while minimizing the test period, with theforce test period being more than 10 times shorter than previous testperiods.

BRIEF DESCRIPTION OF THE DRAWINGS

Specific embodiments of the method according to the invention will nowbe described with reference to the accompanying drawings, in which:

FIG. 1 is a block-diagram of a device for carrying out a methodaccording to the invention;

FIG. 2 shows movement of a test sample and energization anddeenergization of a device during such movement, according to theinvention;

FIG. 3 shows force characteristics of a device obtained using the methodaccording to the invention and prior art;

FIG. 4 is an illustration similar to FIG. 2, but with the use of a priorart method for determining force characteristic.

PREFERRED EMBODIMENT OF THE INVENTION

For determining force characteristic of a device for magneticallyholding workpieces, a test sample or workpiece 1 (FIG. 1) is connectedto a force detector or indicator 2 (i.e. an attraction force pick-up).The workpiece 1 is then placed in parallel with the surface 3 of a topplate of the device cooperating with a source of magnetic field (notshown) with a clearance δ (FIG. 2) which depends on sensitivity of thepick-up 2 (FIG. 1).

The higher the sensitivity of the pick-up 2, the larger is the clearanceδ (FIG. 2). In practice, the clearance may range from hundreths ofmillimeter to several millimeters.

The source of magnetic field remains permanently energized during thetests so that the magnetic force of attraction is continuously provided.The workpiece 1 is continually moved with the pick-up 2 uniformly alonga test line 4 over the surface 3 of the plate. The clearance

between the workpiece 1 and the surface 3 is held substantially constantduring movement of the workpiece 1. The workpiece 1 is caused to move byany appropriate known means.

Readings generated at the output of the force pick-up 2 characterize theforce of attraction of the workpiece 1 to the surface 3 of the plate ateach point over which the workpiece 1 moves. These readings arecontinuously recorded by means of an instrument 5 so that variations inthe force of attraction can be determined. The instrument 5 ispreferably a self-recorder shown at the same reference numeral 5. Anamplifier 6 may be used, if necessary, to provide a signal to theinstrument 5.

During the tests, current I that flows through an electromagnetic coilof the device is held constant during the whole time t of movement ofthe sample 1 placed at a distance δ₁ from the surface 3 of the deviceplate as shown in FIG. 2.

Calibration of the instrument 5 (graduation of its scale in units ofattraction force) is carried out by known methods by stripping a sampleoff the device in the energized position at one or two points along thetest line 4, recording the stripping force and determining readings ofthe instrument 5 when the sample 1 is over these points.

The recorder 5 will continuously record attraction force during thetests and will plot a force characteristic I shown in FIG. 3. FIG. 3shows a plot of specific attraction force P, in N/cm² , as a function ofthe distance S, in cm, through which the workpiece 1 has been movedalong the test line 4.

Numbers of points of stripping of a sample during force tests using theprior art method illustrated by a chart shown in FIG. 4 are plottedalong the S-axis,

When force characteristic of the device is determined by the prior artmethod, the workpiece 1 is placed on the surface of the plate 1 (δ=0) ofthe device which is deenergized, the device is then energized (I×I₁ )and the workpiece is stripped off the surface of the device; the valueof the stripping force is recorded at each point, and these values areplotted in the P and S coordinates and interconnected by a broken lineII (dotted line in FIG. 3).

Comparison of lines I and II in FIG. 3 shows that the method accordingto the invention makes it possible to reveal in the most comprehensivemanner the location of zones of minimum forces of attraction of thesurface 3 of the device plate and to determine accurately the value ofthese forces. This allows machining conditions for workpieces held onthe device to be sufficiently accurately determined and makes itpossible to ensure reliability and safety of the device in operation.

A pilot sample of a device for determining force characteristic ofmagnetic workpiece holders according to the invention was manufacturedfor force testing of electromagnetic plates of a size 200×400 mm with atest sample 10 mm in diameter. The tests showed that the device recordedthe full force characteristic of the plate in 5 to 10 minutes.

Tests of the same plate using the prior art method showed that allvalues of stripping forces applied to the test sample at control pointscoincided with corresponding points of the continuous curve obtainedusing the method according to the invention.

INDUSTRIAL APPLICABILITY

The method according to the invention may be most advantageously usedfor determining force characteristic of devices for magnetically holdingworkpieces installed on grinders, millers and planers as well as onother machine tools.

We claim:
 1. A method for determining a force characteristic of a devicefor magnetically holding workpieces placed on a top plate of the device,wherein a source of magnetic field attracts a workpiece to said topplate of the device, comprising the steps ofconnecting the workpiece toa force indicator which indicates a magnetic force of attraction betweensaid workpiece and said top plate of the device, permanently energizingthe source of magnetic field during a test period to continuouslyprovide said magnetic force of attraction and minimize the duration ofsaid test period, aligning the workpiece so that it is placed inparallel with the top plate, providing a clearance between saidworkpiece and said top plate in an amount which depends on sensitivityof the force indicator, continually and uniformly moving said workpiecealong said top plate along a test line, maintaining the amount of theclearance substantially unchanged, generated a signal at an output ofsaid force indicator in proportion to the magnetic force of attractionthroughout movement of said workpiece along said test line, andrecording said signal continuously so that variations in the magneticforce of attraction can be determined.